Strong porous polytetrafluoroethylene (hereinafter PTFE) products and their method of manufacture involving expansion by stretching were first described in U.S. Pat. Nos. 3,953,566 and 4,187,390. These products have found widespread acceptance in a variety of fields including medical devices, fabrics, electrical insulation, filtration, sealants and packings, and threads and filaments for weaving and sewing. Porous expanded PTFE products have been particularly useful in the field of medical devices because of the highly inert chemical character of PTFE and because the characteristics of the microstructure of the porous expanded PTFE can be controlled to either allow or preclude ingrowth of living tissues. These medical devices include vascular grafts, vascular and hernia patch repair materials, sutures, ligaments and periodontal repair materials. In fabric constructions, porous expanded PTFE materials have been particularly useful because they are both waterproof and breathable. Porous expanded PTFE is particularly useful as an electrical insulating material because of its high dielectric constant. All of these articles using porous expanded PTFE made as taught by U.S. Pat. No. 3,953,566 have a microstructure of nodes interconnected by fibrils.
Heretofore, there have been relatively few applications calling for porous expanded PTFE materials containing macroscopic perforations through the thickness of the material. A paper by J. M. Schakenraad describing GORE-TEX.RTM. Soft Tissue Patch repair material containing perforations made with a 22 gauge needle (25 perforations per square cm) suggests that such a perforated material allows for faster tissue ingrowth than a comparable unperforated material (Improved tissue ingrowth and anchorage of expanded PTFE by perforation: an experimental study in the rat. Biomaterials 1991 vol 12 January). Microscopic evaluation of perforations made in porous expanded PTFE materials that had been expanded prior to perforation reveals that the perforations have extremely rough edges, apparently resulting from irregular cutting and distortion of the material during penetration by the needle. It has been found that alternative methods of making perforations in porous expanded PTFE, such as by removing material with a sharp blade, also result in perforations having rough edges. Further, these methods of forming perforations in porous expanded PTFE result in only slight deformation of the node and fibril microstructure immediately adjacent to the edge of the perforation.
The present invention relates to a macroscopically perforated porous expanded PTFE sheet material having perforations formed prior to the expansion of the PTFE sheet material. The macroscopically perforated porous expanded PTFE sheet material of the present invention may optionally have perforations with substantially smooth edges as opposed to the rough edges resulting from perforations created subsequent to expansion.
U.S. Pat, No. 4,647,416 teaches a method of providing reinforcing ribs on the exterior of a porous expanded PTFE tube by exteriorly circumferentially scoring the wall of a tubular PTFE extrudate prior to expansion of the extrudate. The scoring is required to be of a depth substantially less than the thickness of the wall to avoid weakening and perforating the tube.